Optimize grid filtering and item keys to reduce scroll lag

[michaelbel]

Summary

• cache filtered emoji packs with derived state to reduce unnecessary recompositions while scrolling
• reuse the JSON parser instance and add stable keys to lazy grid items for smoother rendering
• introduce a helper data class to bundle filtered pack data

Testing

• ./gradlew ktlintFormat (fails: task not found in project, requires ktlint tasks to be available)

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Codex Task